The present invention relates generally to the field of television receivers, and, more particularly, television receivers and methods for reducing ghosting and for mitigating the deleterious effects on television signal reception due to multipath propagation.
In the United States (U.S.), there are numerous television stations that transmit video modulated signals from towers distributed about the country. In many locations, it may be difficult or impossible to use a fixed-pointing directional antenna, as the television transmission towers may lie in various directions. In general, there is no reason to expect better angular correlation of sites with the introduction of digital or HDTV standards.
Antenna rotators are often used to overcome this difficulty, but the rotators typically operate slower than the speed at which a viewer may wish to “channel-surf.” The need to re-orient an antenna to receive different channels may be an inconvenience and may present other difficulties when an antenna is shared between many television sets in different rooms or dwellings, such as an apartment building where many different viewers will typically be tuned to many different television channels simultaneously.
In apartment buildings without communal antenna systems, a viewer may need to resort to an indoor antenna to receive broadcast television signals, which may then be subject to distortion due to reflection from nearby objects in the same room or even on the other side of radio-transparent walls. In locations obscured by surrounding high terrain, it may be difficult or impossible to receive a direct television signal wave with any reasonable external antenna mast height, although television signals may be received by diffraction over a hill or by reflection. These diffracted and/or reflected signals may be impaired due to multipath propagation, which may result in attenuation of some frequency components of the video signal and/or ghosting if the multipath delays are a non-negligible fraction of the line scan period. Often, the preference for a high-gain, outdoor, directional antenna stems not from the need for greater signal strength, but from the need to exclude signal reflections by angle-of-arrival discrimination to reduce ghosting.
U.S. Pat. No. 5,119,196 to Ayanoglu et al. (hereinafter '196 patent), the disclosure of which is hereby incorporated herein by reference, describes finite impulse response (FIR) and infinite impulse response (IIR) equalization techniques for television picture ghost cancellation. The '196 patent describes limitations of an IIR channel inverse equalizer when the poles of the channel-descriptive z-polynomial approach the unit circle. The '196 patent also describes factorizing the channel-descriptive polynomial into a causal factor having poles inside the unit circle and an anti-causal factor having poles outside the unit circle, and processing a scan line of signal samples in time-reversed order to implement the anti-causal part. The '196 patent explains that the limitations of these conventional equalizers may be partially eliminated by assuming that the line sync pulse is a zero video signal for a period at each end of the scan line that can be regarded as a guard time between lines. Thus, for ghost delay spread shorter than this guard time, signal equalization may be performed on a line-by-line basis with no carryover from or to adjacent lines. This technique equalizes the video information in the line, but may not remove interference from the sync pulse. Unfortunately, according to the U.S. NTSC standard, the sync pulse is only a zero signal only for a period after detection and if the sync pulse is clamped by a DC restoration circuit such that its peak value is zero. It may be desirable to equalize a television signal predetection at which time the sync pulse is not a zero signal, but instead is a period of maximum transmission.
The techniques described in the '196 patent may also be limited in the delay that they can equalize. The '196 patent describes a processing-intensive technique for adapting equalizer weights for a least-squares reproduction of a test signal. The '196 patent suggests complementing the ghost reduction equalizers with an adaptive antenna, but does not provide implementation details for the adaptive antenna or suggest that the adaptive antenna may be a diversity antenna having two separate output signals. The '196 patent appears to suggest processing the television video signal after detection, which may not allow exploitation of the relative phase difference between delayed signal paths. Finally, the equalization system described in the '196 patent is designed to cancel echos by subtracting them away from the desired signal. Unfortunately, this approach may risk degradation of the desired signal in the echo cancellation process.
Other patents that discuss ghost removal include: U.S. Pat. No. 5,253,063 to Ebihara et al. (hereinafter '063 patent), U.S. Pat. No. 5,812,217 to Cahill, III (hereinafter '217 patent), and U.S. Pat. No. 5,331,416 to Patel et al. (hereinafter '416 patent). Patents that discuss antenna selection diversity include: U.S. Pat. No. 5,335,010 to Lendemeier et al. (hereinafter '010 patent) and U.S. Pat. No. 5,818,543 to Lee (hereinafter '543 patent). The '063 patent, '217 patent, '416 patent, '010 patent, and '543 patent are hereby incorporated herein by reference.
Unfortunately, antenna selection diversity may be hard to reconcile with ghost equalization due to the changing ghost characteristics caused by antenna switching. Accordingly, there is a need for improved television receivers, which may reconcile diversity reception with ghost removal in an economically efficient manner.